The present invention relates to analog signal gain adjustment and equalization, in particular, to analog signal gain controllers and equalizers that adaptively compensate for analog signal distortions due to analog signal transmission through a cable.
As is well-known, data communication via Ethernet (e.g., 100 Mb and 1 Gb per second) is accomplished by transmitting and receiving data bits as analog signals over cables such as category 5 (CAT5) unshielded twisted pair (UTP), with the receiver recovering the digital data bits and clock from the transmitted analog signal. The signals containing the data are typically distorted in signal gain and phase as they are transmitted over long cable lengths at high frequencies. The received signal usually has a higher amplitude than that for which the receiver circuitry is designed. Accordingly, an automatic gain control (AGC) circuit lowers the gain to a level so that the data processing in subsequent circuit stages, such as an equalizer and analog-to-digital converter (ADC), can be done at lower signal levels, and thus at lower power. Additionally, the channel distortion must be equalized or otherwise corrected so as to allow the data clock signal to be properly recovered from the received signal, thereby minimizing the bit error rate (BER) associated with the recovered data.
Such signal equalization is typically done prior to clock or data recovery, with the equalization compensating for the cable loss by reshaping the signal to be closer to its original waveform to minimize intersymbol interface (ISI) within the recovered data signal.
Cable dispersion loss is generally linearly proportional to the cable length and to the square root of the data rate, thereby making the maximum data rate or transmission frequency dependent upon the cable length. Accordingly, as is well known in the art, an adaptive equalizer synthesizes a frequency response that is inversely proportional to the frequency response of the cable. Various adaptive signal equalizers have been developed, including those described in U.S. Pat. Nos. 5,455,843, 5,841,810, 6,545,622, and 6,492,876, the disclosures of which are incorporated herein by reference.